Field of the Invention
The invention relates to a semiconductor body formed of silicon, which can be soldered to a metal substrate through a sequence of metal layers. The invention also relates to a method for soldering the semiconductor body.
Such semiconductor bodies are built into semiconductor components, especially power semiconductor components, great numbers of which are on the market. The sequence of metal layers as a rule includes an aluminum layer, which is seated on a silicon semiconductor body. The aluminum layer adheres well to silicon and forms a satisfactory ohmic contact, especially with p-doped silicon. A diffusion barrier layer which is seated on the aluminum layer in the prior art is usually formed of titanium or chromium and acts as an adhesion promoter and a rear-side barrier between a nickel layer that is seated on the diffusion barrier layer and the aluminum layer. A noble metal layer is applied either directly over the nickel layer in the prior art or over a succeeding thin titanium layer that serves to improve adhesion. The noble metal layer usually is silver, gold or palladium and acts as an oxidation guard for the nickel layer.
It is known from German Published, Non-Prosecuted Patent Application DE-OS 19606101, corresponding to U.S. Pat. No. 5,901,901, to apply a solder material layer onto the diffusion barrier layer. In that reference, the semiconductor body is then typically soldered onto the metal substrate by being applied to the substrate and soldered directly to the substrate, that is without the addition of further soldering agents and fluxing agents, by being heated to temperatures of above approximately 250.degree. C.
A disadvantage of that method and that metal system is that at least one diffusion barrier layer, one adhesion layer, and one solder layer must be used in order to make a usable, solid soldered connection between the metal substrate and the semiconductor body.
German Published, Non-Prosecuted Patent Application DE-OS 19 606 101, in particular, discloses a metal system simplified by two layers, in which in particular one nickel layer that is approximately 1 pm thick is dispensed with, thus considerably reducing the tension in the metal system and therefore causing less wafer warping. In that reference the nickel layer can be dispensed with because the method of "isothermal solidification" is employed as the soldering method, in which the solder layer is so thin, that is approximately 3 .mu.m, that it is entirely converted into intermetallic phases. That limits the incorporation of solder, particularly tin, into the neighboring layer by alloying. The electrical connection with the silicon semiconductor body is achieved with an aluminum layer, as before, and aluminum silicide forms at the interface.